Tap on the starter with a extension or pry bar while someone tries to crank it over (BE VERY CAREFUL NOT TO TOUCH THE BATTERY CABLE CONNECTIONS OR LARGE TERMINALS AT THE STARTER WITH THE TAPPING INSTRUMENT UNLESS YOU WANT TO SEE FIREWORKS!). If it starts replace the starter. If it doesn't, you need to verify that you are getting a 12v signal at the start terminal when cranking. If no signal, then check all fuses and diagnose circuit between ignition switch and start terminal for breaks. You can also jump 12v power to the start terminal at the solenoid to operate the starter. If the key is on and the starter is ok, then it will start.